X-ray Detection

A thickness of 200$\mu$m of GaAs has more than an order of magnitude higher X-ray absorption efficiency with respect to 300$\mu$m of silicon over the medically interesting energy range of 10 to 100keV. The detection properties of GaAs and Ge are similar but GaAs detectors can be operated at or just below room temperature. The increased absorption efficiency implies that a lower dose is required for imaging applications to produce an image of similar quality to that from film or silicon detectors. A 100$\mu$m thick GaAs pad detector with an ion-implanted ohmic contact, biased at 500V, has been used to produce Am-241 X-ray spectra [19]. At 20^oC the energy resolution of 20% allowed lines down to 20keV to be distinguished. At -30^oC the leakage current reduction allowed lines as low as 8keV to be resolved, with a resolution of 4%.

Low pressure growth of VPE GaAs is being investigated as a possible source of affordable, high quality material for X-ray imaging. The first samples have recently been supplied for evaluation by Epitronics Corp. [20]. These are 80$\mu$m thick, with charge carrier concentration $\sim$ 2-3 10¹⁴ /cm³. The material was found to deplete as expected from standard theory to a depth of 20$\mu$m for an applied bias of 100V. The CCE of simple pad detectors fabricated with this material was 100% in the depleted region, allowing the Barium (32keV) K X-ray line to be resolved. The limiting factor was again the leakage current noise which was very high due to the rough surface of the VPE material. Careful mechanical and chemical polishing of the wafer can reduce the leakage current and thus allow better resolution to be obtained.

GaAs pixel detectors bump-bonded to Omega3 read-out electronics have been used to image medical X-ray phantoms with Am-241 and Cd-109 X-ray sources [21]. The results are encouraging, with high signal-to-noise and signal-to-contrast ratios being obtained. The results were improved by the partial removal of Compton interactions using the fast-OR trigger of the electronics. Auto-radiography applications have been investigated by imaging beta particles from P-32 labelled human mammary cells. A low noise level of 10^-4 cps mm^-2 and linearity for activities between 0.005nCi and 0.5nCi were obtained. Further improvements are possible in the quality of both the VPE and LEC material and should allow better results to be obtained.